Underbust+4 vs Underbust+0: Which Bra Sizing Method Should You Use?

Bra Size Calculator Online supports two different ways to turn a snug underbust measurement into a band size: the Underbust+4 method and the Underbust+0 method. The difference is exactly what the names suggest. Underbust+4 rounds your snug underbust measurement to the nearest even number and adds 4 inches to get the band size. Underbust+0 uses the underbust measurement itself, rounded to the nearest even number, as the band size. Both methods then compare that band size to your standing bust measurement to determine the cup size. Which one gives you the right fit depends on where you shop and how the brand builds its bras.

The two methods side by side

The two measurements you need are the same for both methods: a snug underbust (tape comfortably under your bust, perfectly horizontal, firm but not pressing into the skin) and a standing bust (tape parallel to the ground while standing or sitting upright, not squeezing and not hanging loose). Only the band calculation changes.

Why the same body can produce two different sizes

Take a 29-inch snug underbust. Under Underbust+4, you round 29 up to 30 and add 4, giving a 34 band. Under Underbust+0, you round 29 up to 30 and stop there, giving a 30 band. That is a two-band-size gap from identical measurements.

The cup size shifts with it. Cup size comes from the difference between your standing bust and the band size, so a smaller band number paired with the same bust measurement produces a larger cup letter. In practice, a 34B under one method can correspond to a 30D or similar under the other — the same body, described two ways.

Which method should you actually use?

Use the method that matches the brand you are buying from, not the one that sounds more modern or more accurate.

• Shopping on Amazon or from US/UK brands like Natori and Calvin Klein: start with Underbust+4, since that is the system these sellers commonly use to label their sizes.
• Shopping from a brand that sizes close to the body: use Underbust+0.
• Not sure which system a brand uses: calculate both, then check the brand's own size chart before ordering. The band chart and cup chart on Bra Size Calculator Online exist precisely so you can convert between systems rather than guess.

A useful sanity check: the band should feel snug on the loosest hook without riding up, and the center of the bra should sit flat against your chest. If a 34 band rides up but a 32 feels tight, you may be a +0 size being sold a +4 label, or the reverse.

Common mistakes that break both methods

• Measuring the underbust too loose or too tight. The tape should feel the way you want the bra to feel — firm, horizontal, not digging in. A loose measurement inflates the band; an over-tight one shrinks it.
• Measuring the bust while leaning or with a padded bra on. Use a standing or seated upright position and no bra, or a thin unpadded one.
• Mixing units. The calculator accepts inches or centimeters, but you must specify which. Entering centimeters as inches will throw off both methods.
• Assuming one method is "correct." Neither is wrong; they are conventions. The right one is the one the brand uses.

FAQ

Does Underbust+4 always add exactly 4? Yes — the method rounds the snug underbust to the nearest even number first, then adds 4. So 29" becomes 34, and 30" becomes 34 as well.

Can I use Underbust+0 if I normally wear a +4 size? You can calculate it, but expect a smaller band and a larger cup letter. Whether it fits depends on the brand's construction, so try before committing if possible.

Which method does Bra Size Calculator Online recommend? The site presents both and lets you choose, because the correct one depends on the sizing system of the brand you are buying from. Use the band and cup charts to convert between them.